The mobile communication system can meet the demands of people for communicating anywhere and any time and has been developed very quickly since its coming into existence. Along with the proposal and further development of the Third Generation (3G) mobile communication technologies, the bandwidth of the mobile network has been greatly increased, the mobile communication will no longer be limited to the traditional voice communication, and multimedia services combining various multimedia, such as audio, video, picture and text, will be developed gradually. The combination of mobile communication and data services, such as a presence service, a short message service, a web browsing service, a positioning information service, a push service, and a file sharing service, can provide a user with more services, including the message services, such as immediate messaging, chat room, and multimedia short message, video services, such as entertainment, multimedia information, and daily communication, e-business services, such as catalogue, search engine, shopping cart, order management and payment, game services such as single player game and group game, location services such as person searching, guiding and warning, and personal assistant services, such as address book, schedule, bookmark management, file storing, event reminder and e-mail, so as to meet various user demands more properly.
Considering that the IP network has seen increasingly wide application, standardization organizations, such as the 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2), have constituted a standard for developing the mobile network to a whole packet and whole IP network and also have proposed a multimedia subsystem architecture based on the IP for the purpose that uniform standard open architecture is to be used in the mobile network to implement various multimedia applications to provide more choices and more experiences for a user.
An IP multimedia subsystem (IMS) domain which is overlaid on a packet domain network has been introduced at the stage of Release 5 (R5) by the 3GPP. The IMS includes function entities, such as a Call Session Control Function (CSCF), a Media Gateway Control Function (MGCF), a Multimedia Resource Function (MRF) and a Home Subscriber Server (HSS). The CSCF can be further categorized into three logical entities, Serving CSCF (S-CSCF), Proxy CSCF (P-CSCF) and Interrogating CSCF (I-CSCF), according to different functions implemented by a CSCF; the S-CSCF is a service switching center of the IMS, which is used for executing session control, keeping session state, managing user information and generating charging information etc.; the P-CSCF is an access point where a user terminal accesses the IMS, used for registering a user, controlling Quality of Service (QoS) and managing security etc.; the I-CSCF is used for searching a route, such as the interworking in an IMS domain and the interworking between IMS domains, managing the allocation of an S-CSCF, hiding the topology architecture and configuration of a network from an external network and other IMS domains, and generating charging information etc. The MGCF implements gateway control to achieve the interworking between an IMS network and other networks. The MRF provides media resources, such as receiving and playing a voice, encoding and decoding information transmitted between user terminals, and multimedia conference bridge. The HSS is a user database storing the subscription data and the configuration information of users in an IMS network.
Because the architecture of the IMS network has been implemented as independent of the bottom-layer bearer network, the IMS network defined by the 3GPP can also be applied to other packet networks besides the packet domain network defined by the 3GPP, such as a packet network defined by the 3GPP2, a Wireless Local Area Network (WLAN) and a Next Generation Network (NGN), which makes the type of a user terminal and the type of an access network independent of the network. The use of the IMS network is not limited to a network or an application related to the 3GPP, that is, services and applications of other types of access networks and bearer networks can also be implemented by using the IMS architecture.
The Session Initiation Protocol (SIP), which is an IP telephone signalling protocol proposed by the Internet Engineering Task Force (IETF), is used as the signalling control protocol for an IP multimedia session. As implicated in its name, the SIP, which is used for initiating a session, can control the establishment and termination of a multimedia session in which multiple participants take part, regulate and revise dynamically the attribute of the session, such as the demand for session bandwidth, the types of multimedia (such as a voice, a video and a text) to be transmitted, the encoding and decoding format of the multimedia, and the support for multicast and unicast.
The 3GPP, when constituting criterions related to the IMS, specifies expressly that a user terminal must support the communication using the Internet protocol version 6 (IPv6) addresses when an IP multimedia service is implemented in view of the problems in the Internet Protocol version 4 (IPv4), such as the address space being about to be exhausted, the explosion of the route table, and the fact that a large number of IP addresses are needed in the wide application of the IMS and higher requirements have been set for network security, network quality and network mobility in the NGN implementing the amalgamation of multiple media. Regarding function entities in a mobile network, all the function entities in the IMS, which include User Equipment (UE), CSCF, MGCF, MRF and Application Server (AS), must support the IPv6. Because a GPRS Gateway Support Node (GGSN) has an interface with the IMS, the GGSN must support the IPv6, and the GGSN must also support the Dynamic Host Configuration Protocol (DHCP) of the IPv6 to support dynamic address resolution.
Along with changes of the market and development of the technologies, however, the large-scale commercial use of the IPv6 has not attained the speed expected initially and lacks the supports of all the IPv6 based products, so that IPv6 based IMS applications has been postponed. To provide more abundant multimedia services for a 3G user and promote the commercial use of the Wideband Code Division Multiple Access (WCDMA) IMS as soon as possible to attract more users and promote the development of the 3G network. Mobile operators have suggested to the 3GPP that the IPv4 should be supported in implementing the IMS to meet the demand of the early commercial use. At the same time, the Multimedia Domain (MMD), defined by the standardization organization 3GPP2 of Code Division Multiple Access 2000 (CDMA2000) which is another mainstream of 3G, can be implemented based on the IPv4 or the IPv6, and there have been respectable SIP terminals in commercial use which is based on the IPv4. This will result in a problem in the interworking between the IMS of the 3GPP based on the IPv6 and the external network based on the IPv4. Therefore, the 3GPP decides to add a function for supporting the IPv4 to an early IMS product, which enables the IMS to support the IPv6 and/or the IPv4 at the same time. Thus, the 3GPP needs to study how to introduce the IPv4 based IMS, and how to address such issues as the interworking and roaming between the IPv4 based IMS and IPv6 based IMS, and the interworking between IMS and IPv4 SIP applications.
Various possible interworking problems have already been analyzed in the 3GPP technical report (TR) 23.981, including: the IP version supported by a UE, the IP version supported by an IMS network, the IP version supported by a GPRS network, the roaming situation, an end-to-end interworking scenario, IP version interworking involving a service. A Network Address Translation and Protocol Translation (NAT-PT) function and an Application Layer Gateway (ALG) function are employed where the IP version interworking is needed. The NAT-PT function is used for transforming a network address and a protocol, and the ALG function is used for transforming a related address in the application layer.
Because the time delay of a session is increased once the NAT-PT function and the ALG function are used, the QoS is decreased. This makes clear that the less times an IP address is transformed, the less influence is laid on the QoS of the session, and the loads on the devices implementing the NAT-PT function and ALG function as well as the number of network bottlenecks will be reduced. Therefore, when to introduce the NAT-PT function and ALG function needs to be considered in the implementation of the interworking between the IPv4 and the IPv6 so as to minimize the transforming times in the end-to-end interworking procedure.
The existing solution to the interworking between the IPv4 and the IPv6 in the IMS was proposed by the 3GPP. In the existing technical solution, a UE may support only an IPv4 stack, or only an IPv6 stack, or an IPv4 and IPv6 dual stack; likewise, other function entities involving a session building process, such as a peer UE, an AS, a CSCF, a GGSN, can support only an IPv4 stack, or only an IPv6 stack, or an IPv4 and IPv6 dual stack.
Considering that the development tendency of the IMS network is to use IPv6 addresses only, and because existing IPv4 addresses are in short supply, many operators use IP addresses of a private network, which results in that an IPv4 address must be transformed into an IP address of a public network when an interworking with other IP domain networks is performed. Therefore, in the existing technical solution, it is suggested to make the most of an IPv6 address, which can reduce the cases of using the functions of the ALG and the NAT-PT, thereby improving the QoS of the session.
In the version interworking involving a service, the solution to implement an immediate messaging service between a UE supporting the IPv4 and a UE supporting the IPv6, which is suggested by the 3GPP, is shown in FIG. 1. In FIG. 1, UE A supports the IPv4, and because no media component negotiation is involved, UE A directly uses SIP signalling to carry the related information when sending an immediate message to UE B; UE B supports the IPv6, and the home network of UE B is different from that of UE A. At the initiating side, the home S-CSCF of UE A supports the IPv4 and IPv6 dual stack, which directly transforms an SIP message transmitted by using the IPv4 into an SIP message transmitted by using the IPv6, then continues to send the message to the receiver, UE B.
In the case that a UE in an IMS network initiates a session, the procedure of an existing solution to interworking is shown in FIG. 2. In FIG. 2, the IMS-ALG means an ALG function in the IMS network, the TrGW is an NAT-PT function in the IMS network. As illustrated in FIG. 2, the session initiator UE A supports the IPv6 and the home S-CSCF of UE A supports the IPv4 and IPv6 dual stack. If the home S-CSCF of UE A determines, by a mechanism such as the Domain Name System (DNS) query, that the receiver cannot communicate by the IPv6, the S-CSCF replaces information involving the IPv6 version in the SIP message through the interaction with the IMS-ALG and TrGW, then continues the session negotiation with UE B using the IPv4 to set up a media transmission path.
As can be seen from the above, in the existing solution to the interworking between different IP versions, the IP versions supported by the UE of both communication sides, that is, the IPv4 used by the initiator and the IPv6 used by the receiver, or the IPv6 used by the initiator and the IPv4 used by the receiver, are determined first, then according to this precondition, a mapping of information related to IP versions is performed where the ALG/NAT-PT function can be used as early as possible, that is, the home S-CSCF of the initiator UE supporting a dual stack. Those skilled in the art can understand that the IPv6 is used preferentially in the existing technical solution, that is, transformation from the IPv4 to the IPv6 will be performed once the next function entity can support the IPv6.
In the existing technical solution, conditions of the interworking between different IP versions in the IMS are not considered comprehensively. Furthermore, an unnecessary version transformation is likely to be implemented in the interworking between different IP versions, which increases the loads on ALG/NAT-PT devices and thereby affects the QoS of a session. There are two main reasons for this situation. First, it is considered in the existing technical solution that the initiator uses the IPv4 and the receiver uses the IPv6, or the initiator uses the IPv6 and the receiver uses the IPv4, while this is not the case in practical situations. An early IMS in commercial use may support only the IPv4 because of limitations of services and bad usability of IPv6 based devices, while an IMS network which is implemented later on may support the IPv4 and IPv6 dual stack so as to have forward compatibility and backward compatibility, and an IMS network which is implemented when the technologies are more steady and the market is more mature may support only IPv6. Therefore, all the three types of IMS network must be considered when implementing the version interworking, and especially in the early phase of using the IMS network, the interworking with a great deal of the existing IPv4 networks and services also needs to be considered. Otherwise, there may be a great deal of transformation between IP versions which can be avoided essentially. For example, in the case that the home network of a calling UE supports only IPv4, the home network of the called UE supports the IPv4 and IPv6 dual stack, and both the calling UE and the called UE use an IPv4 address, the home network of the calling UE must not perform the transformation from the IPv4 address to the IPv6 address. However, the transformation has to be performed in the existing technical solution.
Second, in practice, the home S-CSCF of a calling UE cannot acquire in advance the type of the IP address which is used by the called UE and can only learn the type of the address of the IP domain where the home network of the called UE is located. In the case that the called UE roams to another network and the IP address of the called UE is assigned by the roaming network, only the home S-CSCF of the called UE knows the IP version which is used by the called UE. According to the processing procedure in the prior art, after a mapping of information related to IP versions is performed where the ALG/NAT-PT can be used as early as possible, a new ALG/NAT-PT transformation may still be needed when a message reaches the home network of the called UE. For example, in the case that the home network of the calling UE supports only IPv4, the home network of the called UE supports the IPv4 and IPv6 dual stack, and both the calling UE and the called UE use an IPv4 address, such transformations are not needed and can be avoided.